The present invention relates to a drive circuit for a voltage driven switching device.
When a semiconductor switching device, such as insulated gate type bipolar transistor (IGBT), is used for driving a motor, and if arm short circuit or short circuit of load is caused, the collector current of the IGBT increases quickly and consequently the IGBT is broken down due to overcurrent or heat. It, therefore, is necessary to detect a short circuit state in a short time. When classifying roughly, there are two major known methods of detecting the short circuit. One is a method, as disclosed in the Japanese Application Patent Laid-open Publication No. Hei 04-79758, where a current sense IGTB is used to detect the short circuit current. The other is a method, as disclosed in the Japanese Application Patent Laid-open Publication No. Hei 02-262826, where the collector voltage is monitored to detect the short circuit state.
On the other hand, there remains a problem that, if current is cut off quickly in a short circuit state, the rate of current change−dI/dt becomes greater and consequently the spike voltage, which is defined by the product of the rate and the stray inductance, becomes greater, resulting in the breakdown of the IGBT. To prevent this, the method disclosed in the Japanese Application Patent Laid-Open Publication Nos. Hei 04-79758 and 02-262826 is provided with a soft cutoff function so as to cut off the main IGBT slowing in the case short circuit is detected.